The invention relates to a method for driving an output stage by pulse-width modulation, which driving serves to supply a unit cooling the output stage during operation.
If electric motors are used to drive cooling units, in particular refrigerating machines of air conditioners or fans in motor vehicles, the cooling can also be used to cool the output stage. Various requirements have to be considered in the case of driving the output stage by pulse-width modulation (PWM)xe2x80x94inter alia with regard to electromagnetic compatibility and noise intrusions at frequencies below the auditory threshold. These requirements are, however, frequently opposed to a high edge steepness or to a low frequency of the pulses, which are desirable in order to achieve the lowest possible power loss in the output stage.
It is the object of the present invention to design a style of driving an output stage, which driving serves to supply a unit cooling the output stage during operation, in such a way that both the said ambient conditions are generally satisfied, and overloading of the output stage at high ambient temperatures owing to a high power loss is avoided.
According to the invention, this object is achieved by virtue of the fact that at a higher temperature of the output stage a style of driving is performed which results in a low power loss in the output stage, and that at a lower temperature of the output stage driving is performed in such a way that the power loss is higher.
The invention is based on the finding that by cooling during operation for the benefit of the ambient conditions it is possible to permit a higher power loss, whereas when there is no cooling, something which is substantially the case in a short run-up phase, it is possible to permit a somewhat more unfavorable environmental response.
A first embodiment of the invention reduces the power loss during the run-up phase by virtue of the fact that at a higher temperature driving is performed with pulses of higher edge steepness, and at a lower temperature it is performed with pulses of lower edge steepness, whereas a second embodiment reduces the power loss during the run-up phase by virtue of the fact that at a higher temperature driving is performed with a lower PWM carrier frequency than at a lower temperature. It is also possible for the two embodiments to be combined with one another.
Depending on the detail of the preconditions, it is possible to provide in the case of the method according to the invention that there is a gradual transition or that a switchover is made between two styles of driving.
A further refinement of the method according to the invention proceeds from the fact that the higher temperatures occur in a run-up phase, and therefore provide that the transition from a style of driving which is provided at a higher temperature to a style of driving for a lower temperature is performed under time control after switching on. As a result, there is no need either for a temperature sensor nor for a circuit evaluating the output signal thereof.
It is provided according to another embodiment that a selection is made between the styles of driving as a function of a measured temperature. Possible temperature increases outside the run-up phase are also taken into account in this case. Already existing temperature sensors can also be used, if appropriate.